Many devices used in medical applications require that the bulk of the device has one property, while the surface of the device has another property. For example, contact lenses may have high oxygen permeability through the lens to maintain good corneal health. However, materials that exhibit exceptionally high oxygen permeability (e.g. polysiloxanes) are typically hydrophobic and will adhere to the eye. Thus, such type of contact lenses typically have a core or bulk material that is highly oxygen permeable and hydrophobic, and a surface that has been treated or coated to increase hydrophilicity, thereby allowing the lens to freely move on the eye and preventing adherence of excessive amounts of tear lipid and protein.
In order to modify the nature of the surface of a relatively hydrophobic contact lens material, a contact lens may be treated with plasma. For example, a plasma treatment technique is disclosed in PCT Publication No. WO 96/31792. Some plasma treatment processes, however, require a significant monetary investment in certain equipment. Moreover, plasma treatment requires that the lens is dry before exposure to the plasma. Thus, lenses that are wet from prior hydration or extraction processes must be dried, thereby adding costs for obtaining drying equipment, as well as added time in the overall lens production process.
In order to overcome the above-mentioned drawbacks of plasma treatment various layer-by-layer (LbL) polyelectrolyte deposition techniques have been developed for modifying the nature of the surface of relatively hydrophobic contact lens materials, (e.g., PCT Publication Nos. WO 01/57118, WO 99/35520). These layer-by-layer techniques effectively alter the surfaces of various materials, such as contact lenses. One layer-by-layer (LbL) coating technique involves consecutively dipping a substrate into solutions of oppositely charged polymeric materials until a coating of a desired thickness and hydrophilicity is formed. Another technique that results in a layer-by-layer coating while avoiding the time-consuming aspects of sequential dipping, is the single dip process disclosed in WO 01/57118, which applies charged polymeric material onto the substrate with only a single dip.
Thus, a number of methods of altering the surface properties of polymeric biomaterials using layer-by-layer techniques have been developed. Nevertheless, although these techniques provide effective deposition of polyelectrolytes to biomaterials, such as contact lenses, a need for further improvements still remains. For example, it has been observed that delamination may occur in coatings having a large number of polyelectrolyte bilayers.
Thus, a need currently exists for an improved method of coating a core material, such as that of a contact lens, with polyelectrolyte (polyionic) layers. In particular, a need exists for an improved polyelectrolyte deposition technique that provides a polyelectrolyte coating with improved durability, i.e. a polyelectrolyte coating that is robust against autoclaving and/or rubbing.